The most efficient power steering gear assembly in production today is the gear having a spool-type distributing valve within the piston. The Sheppard Power Steering System providing this basic concept is first disclosed and claimed in previous U.S. Pat. No. 3,092,083, Sheppard, which patent is exclusively licensed to the same licensee as the invention covered by the present application. This basic design introduced in the early 1960's has been improved upon several times. One of the main areas of recognized improvement is in the area of reducing the frictional forces between the piston and the cylinder and/or the valve and the valve bore within the piston and thereby improving the reversibility of the gear.
A major breakthrough improvement in this area is represented by the Sheppard U.S. Pat. No. 4,088,063, issued May 9, 1978 and also exclusively licensed to the same licensee, R. H. Sheppard Co., Inc., Hanover, Pennsylvania. This generation of gears was first introduced in the middle 1970's and has proved to be highly efficient and effective in providing greater steering efficiency. Truck manufacturers using this generation of gears have been able to appreciably reduce the size and weight of the power steering unit and at the same time gain improved efficiency to handle the larger and larger vehicles carrying the greater and greater payloads of today.
In the gear built under the Sheppard '063 patent, the superior reversibility (change or beginning of movement) of the valve within the piston and the piston within the cylinder results from several unique concepts. First, longitudinal fluid bearing grooves are provided along the side of the piston opposite the pressurized fluid input groove to cushion the piston and balance the opposing forces of the pressurized oil delivered to the power steering gear. The oil between the piston and cylinder provides a separating film between the adjacent like metal surfaces, thus preventing sticking action between the parts and minimizing wear. Additional lubricating ports (advantageously positioned to eliminate unwanted bleeding between the high pressure and the low pressure hydraulic fluid) further reduce friction and generally enhance the reversibility or break loose characteristic of the piston within the cylinder of the gear. Thirdly, repositioning of the guide pin for the valve adjacent the input shaft end of the piston provides improved reversibility of the valve within the piston by obviating the effect of slight deflection of the parts during periods of high dynamic torquing of the steering gear. All of these features, and other features, are fully described and claimed separately, and in combination, in the '063 Sheppard patent.
The desire to further improve the reversibility within the power steering gear thereby bringing about even greater operating efficiencies, lead me to invent the concept covered in the patent application, Power Steering Piston and Cylinder Combination with Plastic Wear Surfaces, Ser. No. 843,976, filed Oct. 20, 1977 now abandoned. This prior invention disclosed and claimed the concept of providing plastic wear surfaces adjacent the ends of the piston to provide an alternative to the fluidized balancing bearings of the '063 patent. The plastic bearing strips are formed of epoxy substantially spanning the distance between the ends of the output rack and the ends of the piston. The concept, which has worked well in use, is a viable alternative to solve the friction problem and to prevent the related slip stick phenomena from arising. The concept relies on the differential molecular structure between the plastic and the metal cylinder wall to prevent slip stick effect; i.e., the different materials prevent tearing and wearing of the parts in this area of highly concentrated frictional forces. In other words, the plastic serves to interrupt the surface of the piston so that the frictionally engaged surfaces at the cylindrical interface do not have matching molecular configurations, thereby preventing the tendency of the surfaces to match and interlock with the deleterious tendency to literally tear away molecules of metal from one part to the other.
During the advanced test program covering the power steering gear with the improvements of the '976 application, as an alternative to the improvements of the '063 Sheppard patent, it was discovered that in addition to the expected increase in efficiency over the original Sheppard '083 design, there was a discovery of an unexpected, even greater efficiency; in the nature of 5-6% greater performance efficiency. Upon close inspection, this was found to occur when the level of the plastics strips was set below the cylindrical surface of the piston. A further careful analysis of the parameters of the invention, that was first brought about by this investigation, thus led me to the present invention. Basically, I have discovered a way to take the best features from the improved piston of the '063 patent and the piston of my improvement patent application '976, and change the oil distribution on and around the piston and/or the valve, to come up with a power steering gear with even greater efficiency than originally anticipated.